Ion exchange chromatography is a useful process in the separation and purification of certain rare earth metals. The process relies on the ability of a column packed with ion-exchange resin to absorb ions of the metal to be separated and purified, and upon the ion exchange which occurs between an eluent passed over the resin and the ions of the metals, which are desorbed and absorbed in order to separate and purify the various metals. Chelate agents are often employed in the separation and purification since the chelate complexes with metals having greater stability (such as heavier rare earths) and they desorb to a greater extent, while those elements having a less stabile complex with the chelate will desorb to a less extent, typically at the end of the band of elements since they do not have to compete with the other stronger complexing elements.
When applied to the purification of scandium, ion exchange chromatography involves packing a column with resin and the solution containing the scandium is passed over the resin. The eluent is then passed over the resin and a band of scandium spreads forward on the column. The amount of scandium at the front of the band is small at first, and then increases, so that a bell-shaped curve of recovery of scandium occurs. At the front of the band there are usually contaminants, such as iron, nickel or copper, and at the back of the band calcium and magnesium, for example, may bleed into the scandium concentrations. When the band reaches the point that the purified scandium has been eluted, the run is stopped, and the scandium stripped from the resin. Scandium is then precipitated from the strip solution. With this conventional ion exchange process, a loss of 10-50% of the processed scandium is not unusual.
The problems with this process include the contamination that occurs in the scandium band. While a chelating agent is useful as the eluent, it contaminates the scandium band. Other metals may also contaminate the band and scandium may be lost in the eluent.
Accordingly, it is an object of this invention to provide high yield recovery of scandium in ion exchange chromatography, while maintaining the purity of the scandium.
It is another object of this invention to decrease contamination of the recovered scandium from ion exchange chromatography.
A further object of the invention is to provide a method of ion exchange purification of scandium which alters the oxidation potential of the eluting solution in order to remove contaminants.
Another object of the invention is to provide for altering the oxidation potential of the eluting solution in ion exchange purification of scandium by the addition of potassium chlorate and ammonium chloride.
A further object of the invention is to maintain scandium on the column resin in ion exchange purification of scandium.
Another object of the invention is to prevent dilution of the scandium band on the column exchange resin.
A still further object of the invention is to promote retention of scandium on the column resin and minimize dilution of the scandium by controlling the temperature, pH and concentration of the eluting solution.
A further object of the invention is to eliminate contamination caused by the eluent.
Yet another object of the invention is to provide for a secondary separation of monovalent cations in the column resin.
An even further object of the invention is to improve recovery of scandium by pumping dilute scandium over the eluted scandium resin column.